Piston rod brake mechanism

A pen-type injection device comprising:

• a housing extending along a general axis,
• a cartridge comprising a cartridge body, a penetrable septum, and a piston, together defining a variable volume reservoir adapted to hold a liquid substance, the cartridge being rotationally fixed with respect to the housing
• a dose expelling mechanism for expelling a dose of the liquid substance from the cartridge, comprising a piston rod structure for advancing the piston, the piston rod structure comprising
• a piston rod threadedly engaged with the housing and adapted to rotate and thereby undergo helical motion relative to the cartridge body during dose expelling, and
• a piston rod foot rotationally locked to the piston rod and adapted to interface with the piston, and
• output structure producing a detectable output at an angular displacement of the piston rod which corresponds to expelling of a predetermined dose increment, a total number of detectable outputs produced during a dose expelling event thereby being indicative of a size of an expelled dose,
• wherein the pen-type injection device further comprises a brake mechanism configured to slow down rotational motion of the piston rod, the brake mechanism comprising a first contact surface forming part of the cartridge body, and a second contact surface arranged, or adapted to be arranged, in contact with the first contact surface and being rotationally locked with respect to the piston rod structure.